1. Field of the Invention
The present invention relates to a betatron with a variable orbit radius, particularly for producing x-radiation in an x-ray inspection system.
2. Description of the Background Art
X-ray inspection systems are used, as is well-known, in the inspection of large-volume articles such as containers and motor vehicles for illegal contents such as weapons, explosives, or contraband goods. In so doing, x-radiation is produced and directed at the article. The x-radiation attenuated by the article is measured by means of a detector and analyzed by an evaluation unit. Therefore, a conclusion can be reached on the nature of the article. This type of x-ray inspection system is known, for example, from European Pat. No. EP 0 412 190 B1, which corresponds to U.S. Pat. No. 5,065,418.
Betatrons are used to generate x-radiation with the energy of more than 1 MeV needed for the inspection. These are circular accelerators in which electrons are accelerated in an orbit. The accelerated electrons are guided onto a target, where upon impacting they produce Bremsstrahlung whose spectrum depends, inter alia, on the energy of the electrons.
A betatron disclosed in German Patent Application No. DE 23 57 126 A1 includes a two-part inner yoke, in which the front sides of both inner yoke parts face each other spaced apart. A magnetic field is produced in the inner yoke by means of two main field coils. An outer yoke connects the two inner yoke part ends distant from one another and closes the magnetic ring.
An evacuated betatron tube, in which the electrons to be accelerated circulate, is arranged between the front sides of the two inner yoke parts. The front sides of the inner yoke parts are formed in such a way that the magnetic field produced by the main field coil forces the electrons into a circular orbit and moreover focuses them onto the plane in which the orbit lies. To control the magnetic flux, it is prior in the art to arrange a ferromagnetic insert between the front sides of the inner yoke parts within the betatron tube.
The electrons are injected, for example, by means of a electron gun into the betatron tube and the current is increased by the main field coil and thereby the strength of the magnetic field. An electric field, which accelerates the electrons in their orbit, is produced by the changed magnetic field. The Lorentz force on the electrons increases similarly simultaneously with the magnetic field strength. As a result, the electrons are held on the same orbit radius. An electron moves in an orbit when the Lorentz force directed at the center of the orbit and the opposing centripetal force cancel each other. The Wideroe condition follows from this:
                              1          2                ⁢                              ⅆ                                                                      ⅆ            t                              <              B        ⁡                  (                      r            s                    )                    >=                                    ⅆ                                                                      ⅆ            t                          ⁢                  B          ⁡                      (                          r              s                        )                              ⁢                          <              B        ⁡                  (                      r            s                    )                    >=                        1                      π            ·                          r              s              2                                      ⁢                              ∫            A                                                          ⁢                      ∫                                          B                ⁡                                  (                  r                  )                                            ⁢                              ⅆ                A                                                          ⁢                  with
In this case, rs is the nominal orbit radius of the electron, A the area defined by the nominal orbit radius rs, and <B(rs)> the magnetic field strength averaged over the area A.
According to German Patent Application No. DE 23 57 128 A1, another coil is arranged around the ferromagnetic insert, which is connected in series with the main field coil and accordingly supplied with current during the acceleration phase. A thyristor circuit achieves that the additional coil at the end of the acceleration phase changes the magnetic field in such a way that the Wideroe condition is no longer fulfilled and thereby the electrons are deflected from their nominal path onto the target.
The disadvantage of the prior-art betatron is the fact that only a small part of the electrons injected into the betatron tube is accelerated to the desired final energy and thus a high efficiency does not result.